Ultrasound imaging systems for medical diagnosis are well known. In general, an ultrasound transducer transmits ultrasound energy into a region of interest in a patient's body. The ultrasound energy is reflected by different organs and different tissue types. The reflected energy is sensed by the transducer, and the resulting electrical signal is processed to provide an image of the region of interest.
Ultrasound imaging can be performed with transducers located externally or internally. To perform internal imaging, an ultrasound transducer is mounted near the distal end of a probe that is sized and shaped for insertion within a body cavity or passage. For example, ultrasound imaging probes have been developed for use in the esophagus. An example of such a probe is a Model 21362, manufactured and sold by Hewlett Packard Company.
Although the power level transmitted by typical ultrasound imaging transducers is on the order of about 0.1 watt, some heating of the tissue adjacent to the transducer occurs. The maximum heating typically occurs at the surface of the tissue directly adjacent to the ultrasound transducer. The surface temperature of the tissue should not exceed about 41.degree. C., or about 4.degree. C. above normal body temperature, to prevent thermal damage to the tissue. It is thus desirable to monitor the surface temperature of the tissue adjacent to the ultrasound transducer during the imaging procedure. If the maximum allowable temperature is exceeded, the transmitted power can be reduced or turned off entirely.
Although the most straightforward approach to temperature measurement is to place a temperature sensor directly at the tissue surface where maximum heating occurs, this technique has the disadvantage that the temperature sensor causes a perturbation in the transmitted and received ultrasound patterns, and thus reduces the accuracy of the image obtained.
In present practice, a single temperature measurement is taken at a point in thermal proximity to the tissue surface of maximum heating but spaced from that surface. The single temperature measurement contains both surface and ambient temperature information in a relationship that depends upon the geometry of the energy emitting element, the transducer housing and the temperature sensor. The ambient temperature is defined as the patient's body temperature in the region of the probe but thermally isolated from the transducer. Since it is impossible to determine both the ambient and surface temperatures from a single temperature measurement, the reading is typically calibrated for a normal ambient body temperature of 37.degree. C. While this technique provides reasonably accurate measurements of surface temperature when the actual ambient temperature is the same as the calibration value, large errors can occur when the ambient body temperature differs from the calibration value, such as may occur during cardiac surgery.
As an example of this problem, consider a transducer calibrated at a normal body temperature of 37.degree. C. and energized to produce a tissue surface temperature of 41.degree. C. The single temperature sensor may measure a temperature of 39.degree. C. When the transducer is placed in a patient whose body temperature is 37.degree. C., the measured temperature will be 39.degree. C. and an equation used for calculating tissue surface temperature will accurately predict the tissue surface temperature of 41.degree. C. When the patient's temperature is reduced to 25.degree. C., a common value during cardiac surgery, and the temperature sensor reads 39.degree. C., the actual tissue surface temperature will be 49.degree. C. The prediction equation predicts a surface temperature of 41.degree. C., since it assumes an ambient temperature of 37.degree. C. Thus, there is an error of 8.degree. C. in the predicted tissue surface temperature.
Probes and catheters having temperature sensors are disclosed in U.S. Pat. No. 4,960,109 issued Oct. 2, 1990 to Lele, U.S. Pat. No. 4,681,122 issued Jul. 21, 1987 to Winters et al and U.S. Pat. No. 4,901,737 issued Feb. 20, 1990 to Griffin et al. None of these patents disclose a technique for measuring the surface temperature of tissue adjacent to a transducer.
It is a general object of the present invention to provide improved methods and apparatus for measurement of tissue surface temperature during application of energy to biological tissue.
It is another object of the present invention to provide improved methods and apparatus for measurement of tissue surface temperature during ultrasound imaging.
It is a further object of the present invention to provide methods and apparatus for differential temperature measurement of tissue surface temperature during ultrasound imaging without perturbing the ultrasound image.
It is yet another object of the present invention to provide methods and apparatus for accurate measurement of tissue surface temperature during ultrasound imaging.
It is another object of the present invention to provide methods and apparatus for accurate control of maximum tissue surface temperature during ultrasound imaging.